The present invention relates to a coherent sidelobe canceller for eliminating interference signals in a radar system, and more particularly to a coherent sidelobe canceller having digital open-loop configuration for sidelobe cancellation computational processing.
Sidelobe cancellers for radar consist of a main, high gain antenna and a plurality of auxiliary antennas each having a gain corresponding to a sidelobe gain of the main antenna and a beam width relatively broader than that of the main antenna. In the sidelobe canceller, signals received from the main- and auxiliary- antennas are linearly coupled by making use of correlation between the main- and auxiliary- antennas in regard to interference signals coming from sidelobe regions of the main antenna to form a certain pattern in the arrival direction of the interference signals, thus suppressing interference waves.
Referring to FIG. 1, there is shown a typical arrangement of a conventional sidelobe canceller. The sidelobe canceller is configured, as a linear array antenna system having (N+1) channels CH.sub.O to CH.sub.N, comprising a main antenna 100-0, a plurality of auxiliary antennas 100-1 to 100-N, receiver circuits 110-0 to 110-N for receiving signals coming from the main- and auxiliary-antennas, resepctively, to produce radar video signals, a weight computing circuit 120 which effects weighting operation in response to (N+1) radar video signals to produce desired weights, multipliers 130-0 to 130-N for multipling radar video signals from receivers 110-0 to 110-N by weight outputs from the weight computing circuit 120, and an adder 140 for summing respective outputs from the multipliers to produce a summed signal on an output terminal 150.
The function of such a coherent sidelobe canceller will be described using a mathematical expression. Assuming now that an input signal of the main antenna is denoted by X.sub.O (t), the number of the auxiliary antennas N, and input signal of the i-th auxiliary antenna X.sub.i (t), and a weighting coefficient for each i-th input terminal W.sub.i, an output signal y(t) is expressed as ##EQU1##
It has been known that a value of the optimum weighting coefficient for maximizing the suppression of the interference signals coming from sidelobe regions of the main antenna is theoretically obtained by EQU W=.mu..phi..sup.-1 S* (2)
where the asterisk (*) denotes the complex conjugate and this notion is applicable to other equations described below in the same sense, W is a weighting coefficient vector, .mu. is a proportional constant, .phi..sup.-1 is an inverse matrix of covariance matrix .phi. of input signals, and S is called a steering vector and in the case of the sidelobe canceller, is usually given by the following column vector: ##EQU2##
Further, the covariance matrix is expressed by, with E being referred to as an expectation value operator, ##EQU3## Thus, by obtaining a covariance E [X.sub.l *(t)X.sub.m (t)] (l, m=0, 1 . . . , N) with respect to an input time series signal X.sub.i (t) (i=0, 1 . . . , N) from N+1 antennas including the main antenna, the interference signals can be ideally suppressed. However, the following problems arise when the weighting coefficient is obtained on the basis of the theoretical equation (1) in a radar.
(1) Since the equation (2) is obtained by performing complex operation, the operational time required for obtaining the inverse matrix.phi..sup.-1 exponentially increases as N increases.
(2) It is required to perform the expectation value operation over a sufficiently long time in a time range within which the characteristics of interference signals do not vary, thus ensuring a predetermined statistical operational processing accuracy. However, if the operational processing for the expectation value is effected for long time, the delay in processing increases, resulting in degradation of response speed with respect to the interference.
For this reason, in the prior art, a technique for calculating the weighting coefficient due to repetitive operation in an asymptotic manner has been widely used. This technique is expressed by, with the repetitive number being as a subscript, EQU W(k)=W(k-1)+f[X(k), y(k-1)] (5)
where X(k) denotes an input signal vector, y(k) an output signal processed by the coherent sidelobe canceller, and f a function, generally determined by X(k) and y(k). Equation (5) means that the weighting coefficient vector is updated so as to more efficiently suppress interferece signals with respect to the input signal vector X(k) at the K-th trial of the present timing by using a weighting coefficent vector W(k-1) at the (k-1)-th trial and y(k-1) representative of uncancelled interference signal to which the coherent sidelobe cancellation processing is implemented with the weighting coefficient vector W(k-1). The operation for weighting the coefficient vector in equation (5) is generally advantageous but the following problems still remain.
(1) Because of repetitive operations, a time for convergence is required, imposing a limitation on the response speed with respect to interferences, thus giving rise to a large impact in practice in a radar requiring instantaneous response.
(2) In a radar, there generally exist reflected signals (which will be called "clutter" hereinafter) from mountains, buildings, rainy clouds, the sea level etc., as well as interference signals. During repetitive operation process, if clutter signal components are contained in an input signal, they will vary as time elapses to make the operation based on equation (4) invalid, thus failing to obtain a desired interference suppression capability.
Further knowledge in connection with the above technique for obtaining weight coefficients due to repetitive operation is given by e.g. IEEE Trans. Antennas and Propag. AP-24: 585-598 (1976) pp 136-149. This sidelobe cancellation is based on closed-loop processing similar to the above system of FIG. 1. This system carries out weighting coefficient calculation by repetitive operational processing based on a feedback operation. Accordingly, in the application of a radar where instantaneous response is required, there is a tendency that the converging time increases. Further, since this system is unable to reject or eliminate clutter singals, the clutter signal is included in the weighting coefficient computational processing in the vicinity of the clutter region, resulting in difficulty in suppressing the interference signal. Furthermore, in the event that there occurs interference in a clutter region, the clutter signal is included in the feedback loop, thus making it difficult to suppress interference signals.
On the other hand, another prior art interference canceller is disclosed in IEE PROC. Vol. 130. Pts. F and H. No. 1. February 1983. This system is based on an open-loop control. Viz., the latter system employs a weighting coefficient calculation due to digital open-loop processing, requiring about 10 to 100 samples for maintaining a desired calculation accuracy. This results in a delay in processing to cause a time difference between the received signal and the weighting coefficient, thus degrading interference signal suppressing performance. Namely, there is no mechanism for delaying the received signal by a processing delay due to weighting coefficient computational processing. Further, since similar to the former close-loop system, there is no mechanism for rejecting clutter signals, the clutter signal is included, in the weighting coefficient computational processing in the vicinity of the clutter region, thus making it difficult to suppress interference singals. Furthermore, an interference signal is likely to be superposed on a clutter signal in the clutter region, leading to difficulty in suppressing the interference signal.